Remove the redundant and poorly worded warning message.

The paragraph above already says, clearly and correctly, that
"However, being completely deterministic, it is not suitable for
all purposes, and is completely unsuitable for cryptographic purposes."

Also we should make any promises about SystemRandom or os.urandom()
being cryptographically secure (they may be, but be can't validate
that promise).  Further, those are actual random number generators
not psuedo-random number generators.

Doc/library/random.rst

@@ -43,12 +43,6 @@ The :mod:`random` module also provides the :class:`SystemRandom` class which
 uses the system function :func:`os.urandom` to generate random numbers
 from sources provided by the operating system.
 
-.. warning::
-
-   The pseudo-random generators of this module should not be used for
-   security purposes.  Use :func:`os.urandom` or :class:`SystemRandom` if
-   you require a cryptographically secure pseudo-random number generator.
-
 
 Bookkeeping functions: